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5G-Tronic/NAG1: The ISS Factor

Making sure the clutch is used properly is almost as important as these other tips for keeping this transmission in gear.
Thursday, January 30, 2014 - 09:00
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The Mercedes-Benz 5G-TRONIC transmission is an electronically controlled 5 speed RWD automatic transmission commonly referred to as the 722.6 transmission. This is a clutch-to-clutch shifting transmission, meaning there are no bands in the shifting process. As one clutch comes off, it needs to be carefully timed with the on coming clutch. Too fast of a release with the off-coming clutch and the shift will transition will consist of a slight engine flare making it feel like a slip. If the on coming clutch applies too quickly, the overlap of the two clutches causes a very brief tie up or bind up, which feels like an abrupt or harsh shift.

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The mechanical make up of this transmission consists of three planetary gear sets: 2 one-way clutch devices called sprags, three driving and three holding clutch assemblies. Several different gear ratios are available with this transmission as well as various amounts of clutch plates within a clutch pack assembly. This gives the transmission the capability to be fitted behind engines as small as a 4 cylinder to as strong as a 12 cylinders.

When Daimler-Benz announced a $36 billion merger with the United States-based Chrysler Corporation in 1998, this transmission eventually found its way into Dodge, Chrysler and Jeep vehicles. It began with the 2005 Chrysler 300 and Dodge Magnum. Afterward, the Jeep Liberty, Wrangler, Grand Cherokee, Dodge Charger, Challenger, Sprinter and Ram truck were fitted with this transmission as well. It is referred to as the NAG 1 transmission, meaning the New Automatic Gearbox Generation 1.

With this transmission being on the road since the mid-90s as well as being in a broad range of vehicles, they’re not strangers to shop these days with several ailments this transmission is noted for.

First one to mention is the TCM gets filled with transmission fluid due to a leak into the solenoid harness connector. By way of capillary action, fluid is drawn up through the wiring filling the TCM. I know that there are transmissions with computers being inside them submersed in transmission fluid, but these external TCMs are not one of them and do not fare well being filled with fluid. Crazy as it sounds, it happens. The remedy is a new sleeve between the internal conductor plate and external harness connector called the Adapter Plug; part number A 203 540 02 53.

Typically the TCM also will need to be replaced and will need to be ordered via the vehicles VIN. Although the adapter plug is supposed to prevent transmission fluid from leaking fluid into the harness connector, it could fail. A real sure way to prevent fluid from ever wicking up the wires again is to expose a piece of each of the wires and drop some solder on the wire. It will take some time to stagger the exposure of each wire, solder, and then properly wrap and cover your work. But it will definitely ensure that the new TCM you put in will not get loaded with fluid again.

Some risk cleaning out the contaminated TCM to reuse it. In some cases, it has actually worked out for them. But the majority of times it usually comes back with various codes. I am almost tempted to share with you one of the craziest methods I have ever heard being used by some individuals to clean these TCMs. But have decided against it. Too many wives might find oily residue on their dishes after removing them from their dishwasher.

The internal conductor plate just previously mentioned is another commonly known issue with this transmission. It is mounted on the valve body, which the solenoids bolt down onto. Circuitry within the conductor plate runs from each solenoid to the plug connection. Integral to this conductor plate is the starter interlock contact, transmission oil temperature sensor, the N2 and N3 speed sensors. This conductor plate is known to malfunction producing various solenoid and/or speed sensor electrical codes. The part number to replace it is, A 140 270 11 61.

This conductor plate brings up a few very interesting points to mention, which is really the primary focus of this article. Because this plate can fail in such a way where it will produces speed sensor or solenoid codes, there has been at times confusion with diagnosing the N2 and N3 speed sensors.

The N2 and N3 speed sensors together act as the Input Shaft Speed Sensor. The N3 speed sensor (solid black) is the most forward speed sensor on the conductor plate. This sensor reads the rotation of the K1 clutch drum inside the transmission. This drum has the front planetary sun gear attached to it and it lugs into the B1 clutch on the pump. What this means is that whenever the B1 clutch is applied it holds this drum stationary. This occurs in Park, Reverse Standard Mode, 1st and 5th gear. In these ranges the N3 rpm reading will be 0.

The N2 is the next sensor back on the conductor plate (silver top) and reads the rotation of the front planetary carrier assembly. This gets driven by the front ring gear attached to the K2 clutch drum and input shaft. This means that anytime the input shaft is in rotation so is the front planetary carrier assembly. The N2 will provide and rpm reading in park and neutral, and all forward and reverse gear movement but will read 0 rpms when in gear with the brakes applied.

As a side note, the N2 rpm reading in 1st and 5th gear will not be actual input shaft rpm. This is due to the carrier being driven by the input shaft through the internal ring gear, which will slightly reduce the speed of the carrier.

But when the K1 clutch applies, it locks the sun gear to the carrier forcing the entire front planetary assembly to rotate at actual input shaft speed. This occurs in 2nd, 3rd and 4th gears and when it does, both the N2 and N3 rpm sensor readings are exactly the same (if the transmission is functioning correctly).

Chrysler has offered a speed/calculation chart that is provided for you in Figure 10 along with the following explanation: Since the N3 is inoperative in reverse standard mode, 1st and 5th gears, it appears that the N3 sensor is redundant and provides no additional information. However, the TCM needs to know input shaft speed. Since the N2 sensor is not directly tied to the Input Shaft, the N3 sensor is required to inform the TCM to divide the N2 value by 0.6081 or 1.0000 to calculate actual input shaft speed.

The chart in Figure 10 confirms the normal operation of the speed sensors, the N3 sensor being at 0 in reverse (S), 1st and 5th gears while the N2 is providing an rpm reading. Then both are the same in 2nd, 3rd, and 4th.

This statement Chrysler provides in accompaniment with this chart reveals some computer math (strategy). The computer is programmed to divide the 0.6081 value into the N2 rpm reading to obtain a “true” Input Shaft speed in rpm. Once this is calculated, the Output Speed RPM is then used to be divided into the Input Speed to calculate gear ratio.


Figure 11 is a first gear screen capture at 10mph of a 2008 Jeep Grand Cherokee which provides the following data: 

Turbine (Input) Speed: 1,499 rpm

Input Speed Sensor 1 (N2): 912 rpm

Input Speed Sensor 2 (N3): 0 rpm

Output Speed: 421 rpm

The N2 rpm of 912 is divided by the 0.6081 value equating into an ISS rpm speed of 1499.7533. Round that off to 1500 and divide the output speed rpm reading (421) into this number to calculate the current gear ratio which = 3.56 (first gear ratio).

Interestingly enough, the advertised gear ratios for this vehicle are as follows:

1st Gear: 3.59:1
2nd Gear: 2.19:1
3rd Gear: 1.41:1
4th Gear: 1.00:1
5th Gear: 0.83:1
Reverse: 3.16:1

Remember, 3.56 is very close to 3.59 especially when you take into account that scan data can be sluggish, but the figure does reveal a good working unit.

The next interesting aspect to this equation is the Output Speed. This transmission does not have an output speed sensor. The ABS module provides a vehicle speed signal for the instrument cluster and the TCM. The TCM receives this information and via a “pinion factor” it calculates that information into an output speed signal. A pinion factor takes into account the transmission gear ratio, tire size and differential ratio for the TCM to properly calculate an output speed signal. This value is then used to monitor gear ratio health of the transmission by the computer.

What this means to you and me is this. For Dodge, Chrysler and Jeep vehicles, gear ratio errors can be the result of a slipping transmission, an incorrect ratio planetary, an incorrect ratio differential, the wrong TCM is being used or, the wrong pinion factor was programmed into the TCM. All of these factors apply with Mercedes vehicles as well except programming the wrong “pinion factor” as this is not an offered feature.

In many ways this pinion factor can be related to the old DRAC or PSOM. They too needed to be properly calibrated to accommodate the various vehicles they would be used in.

Another misunderstood scan tool parameter that occurs is with the Transmission Fluid Temperature located in the electrical conductor plate.  Its purpose is to measure the fluid temp and pass that information to the TCM as an input signal. The TFT sensor is wired in series with the starter interlock contact also referred to as the Park/Neutral contact. 

The fluid temperature signal is transferred to the TCM only when the dry-reed contact of the Park/Neutral contact is closed. This occurs in Reverse or a forward gear positions only. When in Park or Neutral, the TCM uses engine temperature.  When it becomes necessary to test the TFT sensor’s electrical circuit, the ignition key will need to be placed in the on position with the engine off. The selector lever can then be safely placed into Reverse or Drive to properly conduct the test. In Park or Neutral the circuit is disrupted by the Park/Neutral contact.

With some Mercedes vehicles, there have been reports of transmission fluid being accidently poured into the engine oil fill hole after repairs. Unaware, the vehicle is started with a dry transmission sump. If this is not realized soon enough, the starter interlock/Park/Neutral contact overheats and burns out. This not only damages the conductor plate, it also causes a no start condition. 

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